Spectrum sharing is thought of as one of the most viable ways of improving the amount of spectrum available to wireless networks and other radio devices for conducting wireless communications. But regulations hold that radios using shared spectrum in a permitted area must not cause impermissible interference in an adjacent protected area. Since emissions of multiple radios that share spectrum may aggregate, there is concern that impermissible interference may readily result in certain sharing environments.
Regulators have begun to consider this situation. For instance, in the U.S., the Federal Communications Commission (FCC) adopted “Report and Order and Second Further Notice of Proposed Rulemaking” under FCC 15-47 on Apr. 17, 2015 (released Apr. 21, 2015). FCC 15-47, in part, states:                Received Signal Strength Limits        191. Background. In the FNPRM, we indicated that the SAS should have a baseline threshold for the maximum permitted aggregate signal level from all CBSDs at the borders of PALs. We stated that Citizens Broadband Radio Service users should ensure that the aggregate signal level from their CBSDs as well as the aggregate transmissions from their associated End User Devices at the edge of their authorized service boundaries remain at levels that would not harm other CBSDs in the same or adjacent service areas. For small cell networks, industry standards and studies have shown, so long as interference rise over noise (IoT) remains at or below 20 dB and 55 dB for picocells and femtocells, respectively, performance is not impaired. Based on the industry studies, and taking into account reasonable distance between authorized user operations, we proposed a maximum aggregate signal level threshold of −80 dBm with reference to a 0 dBi antenna in any 10 megahertz bandwidth, at a height of 1.5 meters above the ground level, anywhere along the boundary of a PAL license area. We also proposed to allow neighboring PALs to coordinate and mutually agree on higher or lower signal level thresholds. We sought comment on these proposals.        195. Discussion. After a thorough review of the record, we believe that establishing a baseline maximum signal level along license area boundaries will help foster effective coexistence in the 3.5 GHz Band. We also find that licensees should be permitted to agree to lower or higher acceptable maximum signal levels appropriate to their particular network configurations. We believe that the aggregate −80 dBm per 10 megahertz signal threshold at the service boundaries proposed in the FNPRM is wholly appropriate for the dense cell deployments and relatively small license areas that we expect in this band. Therefore, we adopt our proposal for aggregate received signal level at a PAL license boundary to be at or below an average (rms) power level of −80 dBm when integrated over a 10 MHz reference bandwidth with the measurement antenna placed at a height of 1.5 meters above ground level. We also recognize that the PAL licensees may agree to an alternative limit besides −80 dBm at their service boundaries and communicate it to an SAS. Moreover, these signal level requirements will not apply to adjacent license areas held by the same Priority Access Licensee.        We [the FCC] recognize that ensuring compliance with this limit at the boundary is likely challenging on a real-time basis and there are legitimate questions relative to how to develop appropriate predictive models. We also recognize that the use of an aggregate metric could be challenging in a multi-user environment. We encourage any multi-stakeholder group formed to address technical issues raised by this proceeding to consider how this limit should be applied. As an initial matter, we will apply the limit through measurements at the license area boundary at times of peak activity.        